1. Field of the Invention
The present invention relates to an image forming apparatus and method, particularly to an image forming apparatus utilizing electrophotography such as a copying machine, a printer, a facsimile and a complex machine of those and its image forming method, more specifically, to an image forming apparatus having a hybrid developing apparatus which develops a latent image to which only charged toner is caused to jump from a developing roll holding the only charged toner on it using two components developer material, in which non-magnetic toner is charged by means of magnetic carrier.
2. Description of the Related Art
Heretofore, there are kinds of developing methods in an image forming apparatus utilizing electrophotography such as a copying machine, a printer, a facsimile and a complex machine of those. One is a two-components developer material method which uses toner and magnetic carrier; another is one-component developer material method which uses nonconductive toner or conductive toner, yet another is a hybrid developing method which develops a latent image to which only charged toner is caused to jump from a developing roll holding the only charged toner on it using two components developer material, in which non-magnetic toner is charged by means of magnetic carrier.
A two parts developer material method has advantages of such as excellent charging property of toner by carrier, long life of toner as well as uniformity of solid image. On the other hand, it has drawbacks of such as a large and complicated developing apparatus, generation of toner scattering or carrier accompanying as well as varying image quality depending on durability of carrier. A one-component developer material method has advantages of a compact developing apparatus as well as good dot reproducibility while it has drawbacks of the poor durability of a developing roll and a charge roller, which results in selective developing or high maintenance cost of supplies and material for developing apparatus. A hybrid developing method is a high rate image forming method which has good dot reproducibility and a long life, however it has heretofore such problems as generation of developing ghost or toner scattering.
Though a hybrid developing method has been developed as one of measures for a one-component non-contact developing, it has been recently developed as a high rate developing method for a developing method of superimposing multi-colors on one drum, in which a plurality of color images are sequentially formed on one latent image bearing body (a photosensitive body). Attention has been paid to this method as an art of quality color images because a color image with scarce color drift can be obtained by accurately overlapping different colored layers of toner on a latent image bearing body.
However, since developing apparatuses corresponding to a number of colors used should be disposed around the latent image bearing body in a developing method of superimposing multi-colors on one drum, the latent image bearing body becomes large in size so that this prevents from designing a compact image forming apparatus. Hence, attention has been paid to a tandem method in which a plurality of electro-photographic process units corresponding to colored toners used are aligned and colored images are formed synchronized with transferring of a transfer member so as to overlapping the colored images on the transfer member. However, this method has a drawback that the image forming apparatus becomes large because electro-photographic process units of kinds of colors should be aligned, though it has an advantage in having a rapid rate of printing. In order to improve measures for this problem, a compact tandem image forming apparatus has been proposed, which is provided with a small-sized image forming unit by narrowing a distance between latent image bearing bodies.
The compact tandem image forming apparatus thus designed has an advantage when it has a vertical developing apparatus in order to minimize a size in width direction of the image forming unit. That is, it is desirable regarding parts layout to dispose a developing apparatus in the upper part of the latent image bearing body. However, in case the developing apparatus is disposed in a vertical manner in a conventional two-components developer material method, flow of developer material, that is, the supply of developer material from a developer mixer to the part adjacent to a latent image bearing body is complicated so that down-sizing of the apparatus is limited, carrier adheres inevitably to the latent image bearing body and toner scatters unavoidably.
Though, as another method, a one-component developer material method in which carrier is not used is proposed, it has a drawback of enhancing color drift, which is the weak point of a tandem type, on account of the torque fluctuation of a latent image bearing body caused by contacting a developing roll to the latent image bearing body. Meanwhile, with a method in which a latent image bearing body is not touched, toner is charged by a charge roll and the toner thickness on the developing roll is restricted by an elastic doctor blade. Thus, additive for toner adheres to the charge roll so that the charging ability is decreased or toner adheres to the doctor blade so that toner thickness becomes uneven, which results in image defects.
Hence, as a measure for resolving the problems, the aforementioned hybrid developing method has attracted attention. Though the hybrid developing method has the problems with regard to generation of ghost and scattering of toner, the method has little torque fluctuation owing to non-contacting between the developing roll and the image bearing body as well as excellent dot reproducibility so that it provides a high-speed image forming apparatus capable of a long life.
A prior art regarding a hybrid developing method is disclosed in U.S. Pat. No. 3,866,574 which proposes a method wherein a non-magnetic toner thin layer is formed on a donor roll (developing roll) disposed untouchably toward a latent image bearing body and the toner is caused to jump to the latent image on the latent image bearing body by an alternative current electric field. While, U.S. Pat. No. 3,929,098 shows a developing apparatus which forms a toner layer by transferring the toner onto a donor roll using a magnetic roll.
However, although these arts enable toner to form a thin layer on a donor roll by means of two-components developer material, a strong alternative current electric field is necessary to separate toner from the donor when the electrostatic charge of the toner is high. The strong alternative current electric field disturbs the toner layer so that a problem concerning superimposing multi-colors arises. In this regard, so called a powder cloud developing method is disclosed in Japanese laid open patent publication, No. JP1991-113474 wherein auxiliary electrodes consisting of wires are provided between a donor roll and a latent image bearing body applying a weak alternative current electric field so as not to disturb the toner developed.
Theoretical aspect of the formation of toner layer on a developing roll using two-component developer material was reported by Toshiba Corporation in the Electrophotography Society Journal 19, 2 (1981) and the art is disclosed in Japanese laid-open patent publication JP1984-121077.
The aforementioned prior arts have a tendency of decreasing image consistency owing to selective developing as coarse particles of the high developing natured toner are apt to selectively jump to the latent image body and fine particles of the highly charged toner remains on the developing roll sleeve. Further, the control of charging toner is complicated so that a high surface potential and a big developing electrical field need to be applied. In this regard, if both toner consumption region and non-consumption region are generated on the developing roll, toner adhering state and toner potential vary on the surface of the developing roll. Thus, as shown in FIG. 3, so called a hysteresis, a phenomenon in which a part of previous developing image appears as a ghost is liable to generate. In FIG. 3, 35 is a solid image consisting of a rectangular black image and 36, 37 are half tone image succeeding and broader than the solid image. When both toner consumption region and non-consumption region are generated on the developing roll, printing the half tone images 36, 37 after the solid image 35 generates the ghost shown in FIG. 3(b). Further in case high consistent images are printed repeatedly, unevenness of image such as mottle in image consistency is liable to occur so that a problem in down sizing the apparatus arises.
In order to overcome this problem, Japanese laid-open patent publication JP1999-231652 discloses a member for scraping the residual toner on a developing roll and a recovery device for the scraped toner, while Japanese laid-open patent publication JP2000-81788 discloses an apparatus using an exclusive recovery roll for assuring recovery of toner on the developing roll. However these methods are necessary for complex structures enabling to make practical for a compact electrophotographic apparatus. Further, Japanese laid-open patent publication JP1995-128983 discloses an apparatus wherein toner on a developing roll is recovered by setting a broad half value region of magnetic density of a magnetic roll as a measure for the hysteresis when using magnetic brushes. Japanese laid-open patent publication JP1988-249164 discloses a control method for a developing apparatus of tandem type wherein degradation of developer material is prevented by bringing down the operation of developing apparatus in an image forming part except for the image forming part whereat a transfer process is performed.
In a hybrid developing method, decreasing of image consistency owing to selective developing or developing defect, image degradation, developing ghost, toner scattering and sleeve adhesion owing to leaving for a long time while holding toner on a developing roll are generated. In order to solve the aforementioned problems, Japanese laid-open patent publication JP11994-67546, JP11995-72733 and JP11995-92804 disclose an image forming apparatus having a magnetic roll forming magnetic brushes by two-components developer material, a donor roll (developing roll) holding a toner thin layer supplied by the magnetic roll and electrodes disposed between the donor roll and a latent image bearing body wherein bias consisting of alternate current and direct current voltage are applied to the electrodes, direct current bias is applied to the developing roll and polarity reversible direct current bias by a switch (JP1995-72733) or alternate current bias superposed on direct current (JP1994-67546 and JP1995-92804) is applied to the magnetic roll.
In a developing apparatus described in JP31994-67546 or JP31995-72733, a toner thin layer is formed on the developing roll with magnetic brushes generated on the magnetic roll with direct current potential difference (JP1995-72733) or alternate current bias (JP1994-67546 and JP31995-92804) between the magnetic roll and the developing roll and further a latent image on the latent image bearing body is developed by forming toner cloud near the electrodes with an alternate current bias superposed on direct current applied between the developing roll and the electrodes while the toner on the developing roll is recovered by applying a direct current bias in the direction of peeling toner from the developing roll to the magnetic roll with switching after formation of images (JP1995-72733 and JP1995-92804) or at a definite interval (JP1994-67546) and by applying a direct current bias (JP1995-72733) in the direction of transferring toner on the magnetic roll to the developing roll with switching at the successive image formation, whereby formation of images are prepared to solve the above mentioned problems.
In JP2000-250294, it is argued that a method in which electrodes are provided between a developing roll and a latent image bearing body disclosed in JP1995-72733 has such draw back as an uneven phenomenon owing to vibration of wires tensed with electrical bias and generation of scars on the developing roll by dust attached instantaneously on the electrodes. A hybrid developing apparatus having electrodes buried in a developing roll is introduced as a prior art. Even in the apparatus using a developing roll with electrodes buried in the roll, carrier attached on the developing roll sticks to images and toner is not effectively supplied to the latent image bearing body because electrodes buried in the roll have a certain distance between them. Consequently, the apparatus generates a phenomenon of empty images when images of high image ratio are continuously printed or a phenomenon of low quality image or low image consistency caused by selective development on account of alternate current bias applied to the magnetic roll and the developing roll so that the patent publication disclosed a hybrid developing apparatus preventing these phenomenon.
Namely, in the patent publication JP2000-250294, a dielectric layer into which electrodes are buried with a small distance is provided on a electroconductive sleeve to constitute a developing roll whereby improving the effectiveness of supplying toner to a latent image bearing body, preventing short-circuiting between electrodes and protecting the surface. In addition, an electrode-relaxation-allowing layer having a dielectric constant with which cumulated electric charges can be diffused and capable of fringe charge penetrating through the coating for a shorter time than several seconds is provided. A brush electrode supplying a bias voltage consisting of alternate current voltage and dielectric current voltage to the electrodes buried in the developing roll is provided between the developing roll and the latent image bearing body and another brush electrode supplying another bias voltage consisting of alternate current voltage and dielectric current voltage to the electrodes buried in the developing roll is provided between the developing roll and magnetic roll, so that, a latent image on the latent image bearing body is developed with a cloud of toner between the latent image bearing body and the developing roll, and toner is reciprocated between the developing roll and the magnetic roll. The alternate current voltage source is common to the both biases and its wave is a square wave, a duty ratio of which is set in such a manner that a time for transferring toner from the magnetic roll to developing roll is shorter than a time for recovering toner from developing roll to the magnetic roll so as to prevent selective transfer of toner and carrier sticking to the developing roll utilizing the difference of inertia between toner and carrier.
The powder cloud developing method disclosed in the Japanese laid open patent publication, No. JP1991-113474 is not commonly practiced because weirs of auxiliary electrodes are apt to become dirty and image degradation generates owing to vibration. Further, the apparatuses disclosed in JP1999-231652, JP2000-81788 and JP1995-128983 have following drawbacks; namely, one needs a scratching device for toner and a recovery roll; another has a cause of degradation of toner durability owing to increasing toner stress by applying a special bias for recovery; yet another needs a long time for forming the layer on the developing roll at the time of developing reducing a developing speed. Further, after processing for a long time, charging property of toner varies greatly on the developing roll owing to degradation of durability of carrier leading to broadening the charging distribution of replenishing toner or recovered toner, which results in toner scattering or image fogging caused by poor toner charge. In addition, as the art needs a troublesome exchange of carrier, it does not have turned to a practical use.
In a tandem type developing apparatus disclosed in JP11988-249164, the action of developing apparatus except for an image forming part processing a transfer process needs to be stopped or the apparatus needs an apparatus or an control device for changing a high voltage applied between the developing roll and the magnetic roll with a high frequency resulting in cost up, while the apparatus has a structure of disposing donor roll, a magnetic roll and a stirrer member in a horizontal way so that downsizing of the apparatus is difficult.
An apparatus disclosed in JP1994-67546, JP1995-72733 (U.S. Pat. No. 5,341,197), JP1995-92804, or JP2000-250294 is operated by the powder cloud method in which the wires of the auxiliary electrodes are likely to get dirty and image degradation caused by vibration occurs; toner is recovered from the developing roll to the magnetic roll after an image is formed and a DC bias of which direction cause toner to transfer from the magnetic roll to the developing roll is applied by switching at the next image forming process; the toner layer becomes thinner by the right amount for one round of developing roll when toner is resupplied from the magnetic roll to the developing roll so that the image consistency of an image top end decreases to the extent of the toner layer thickness. This phenomenon occurs not only when toner is resupplied to the developing roll but also when developing is begun.
The apparatus disclosed in JP2000-250294 needs to bury electrodes into the developing roll and has a complicated and costly structure as necessary for a brush electrode for supplying the electrodes a superposed bias of AC and DC. The electrodes disposed intermittently in the circumferential direction while toner can not be controlled when the brush electrode becomes unable to contact the electrodes on the developing roll owing to sticking of toner caused by vibration or getting dirty for some reasons.
Further, as mentioned above, when highly consistent images are printed successively, fine particles of toner and contamination of toner constituents stick to the developing roll that causes toner filming leading to unevenness of images such as generation of heterogeneous image consistency. These are problems when downsizing the developing apparatus.
The present invention was done for solving the above-mentioned problems. An object of the present invention is to provide a compact, rapid and inexpensive image forming apparatus and method of the same having a hybrid developing apparatus wherein developing ghost and selective developing are avoided without complicating a developing apparatus and surly charged toner is supplied to a developing roll while stable quality images are obtained for a long time.
In order to attain these objects, according to the present invention, an image forming apparatus having a hybrid developing apparatus comprising a conveying body of two components developer material, which electrostatically charges developer material consisting of carrier and toner magnetically held thereon, and a developing roll, which forms an only toner thin layer on its surface by transferring the toner from the conveying body wherein latent images on a latent image bearing body are developed with the toner by applying developing bias to nearest approached regions (developing regions) between the developing roll and the latent image bearing body, is characterized in that a developing roll, which is electrically conductive at least on its surface, is directly confronted with a latent image bearing body through an insulative toner thin layer, and that a developing bias is applied to the electrically conductive surface of the developing roll, forming a continuous plane of electrode.
Thus, by making a hybrid developing apparatus with a simple structure in which a developing bias is applied to the developing roll having an electrically conductive surface, complicated structures such as providing an auxiliary electrode or burying electrodes in the developing roll is not necessary. Hence, such trouble as the image degradation owing to the vibration or staining of the wire electrodes or as uncontrollable toner because a brush electrode cannot be touched with the electrode of the developing roll is avoided. Since the surface of the developing roll is formed with a continuous electrically conductive electrode plane and a bias is applied directly to the developing roll so that developing becomes maximum at the nearest region between the developing roll and the latent image bearing body, and developing comes to an end as the distance increases after that, and toner which causes fog is recovered to the developing roll developing accurately the image on the latent image bearing body, a compact image forming apparatus having a hybrid developing apparatus which prevents fog and gives stable images for a long time can be provided.
Further, according to the present invention, an entire developing roll is electrically conductive and an alternate current bias superposed by a direct current bias thereon is applied onto the developing roll.
Thus, by applying an alternate current bias superposed by a direct current bias thereon on to the developing roll which is entirely electrically conductive, an excellent developing property between the latent image bearing body can be obtained and toner can be easily exchanged between the developing roll and the conveying body of developer material so that an image forming apparatus having a hybrid developing apparatus preventing selective developing and giving stable image quality can be provided.
Further, according to the present invention, a duty ratio of the alternative current bias is set as within the range from 10 to 50%.
Thus, by setting a duty ratio as within the range of 10 to 50%, a peak of an alternative current bias becomes sharp so that toner can be effectively recovered and recovery effect from the developing roll to the conveying body of developer material is enhanced so that toner staining of the developing roll is eliminated, and at the same time, toner is recovered effectively from the latent image bearing body to the developing roll so that fog is not likely to appear, whereby an image forming apparatus having a hybrid developing apparatus giving stable image quality can be provided.
Further, according to the present invention, the surface of the developing roll consists of an approximately uniform electro conductive sleeve having a conductivity of 106 xcexa9xc2x7cm3 or less.
Thus, since the surface of the developing roll consists of an approximately uniform electro conductive sleeve having a conductivity of 106 xcexa9xc2x7cm3 or less, the superposed AC, DC bias applied to the developing roll acts favorably between the rotating developing roll and the latent image bearing body and the magnetic roll so that image consistency does not decrease, leading to obtaining excellent images for a long time.
Further, according to the present invention, an image forming apparatus having a hybrid developing apparatus comprising a magnetic roll for conveying two components developer material, which electrostatically charges developer material consisting of carrier and toner magnetically held thereon, and a developing roll, which forms an only toner thin layer on its surface by transferring the toner from the conveying body utilizing a bias for transferring and magnetic brushes of the magnetic roll wherein latent images on a latent image bearing body are developed with the toner by applying developing bias to nearest approached regions (developing regions) between the developing roll and the latent image bearing body, is characterized in that both of the magnetic roll and the developing roll are electrically conductive at least on their surfaces, and that an independent bias is applied to each of the two rolls.
Thus, a hybrid developing apparatus is constructed with the simple structure wherein an independent bias is applied to each of the magnetic roll and the developing roll both of which have electrically conductive surfaces on them so that a complicated structure such as providing an auxiliary electrode or burying electrodes in the developing roll is unnecessary and therefore such trouble as the image degradation owing to the vibration or staining of the wire electrodes or as uncontrollable toner because a brush electrode cannot be touched with the electrode of the developing roll are avoided. Since the surface of the developing roll is electrically conductive and a bias is applied directly to the both rolls so that developing becomes maximum at the nearest region between the developing roll and the latent image bearing body, and developing comes to an end as the distance increases after that, and toner which causes fog is recovered to the developing roll developing accurately the image on the latent image bearing body and recovering the toner on the developing roll with these biases, a compact image forming apparatus having a hybrid developing apparatus which prevents fog and gives stable images for a long time can be provided.
Further, according to the present invention, a bias for conveying applied to the magnetic roll is a direct current bias and a bias for developing applied to the developing roll is a bias consisting of a direct current bias and an alternate current bias superposed thereon.
Thus, by applying a direct current bias to the magnetic roll for a bias for conveying and a bias consisting of a direct current bias and an alternate current bias superposed thereon to the developing roll for a bias for developing, an excellent developing property between the latent image bearing body and the developing roll can be obtained with a alternative current bias applied to the developing roll and toner can be easily exchanged between the developing roll and the magnetic roll so that an image forming apparatus having a hybrid developing apparatus preventing selective developing and giving stable image quality can be provided.
Further, according to the present invention, a direct current voltage of the bias for developing applied to the developing roll is fixed and a direct current voltage of the bias for conveying applied to the magnetic roll is varied on the condition as to whether it is the developing period or the non-developing period (no paper period) within the same polarity.
Thus, by fixing a direct current voltage of the bias for developing applied to the developing roll and varying a direct current voltage of the bias for conveying applied to the magnetic roll on the condition as to whether it is the developing period or the non-developing period (no paper period) within the same polarity, exchange of toner on the developing roll can be controlled without affecting developing of a latent image on the latent image bearing body so that an image forming apparatus having a hybrid developing apparatus preventing selective developing and giving stable image quality for a long time can be provided.
Further, according to the present invention, an angle between a magnetic pole on the magnetic roll nearest to the developing roll and each of both magnetic poles adjacent to that pole on the magnetic roll is 30 degrees or less respectively.
Thus, since an angle between a magnetic pole nearest to the developing roll and each of both magnetic poles adjacent to that pole on the magnetic roll is 30 degrees or less respectively, magnetic brushes between the magnetic pole nearest to the developing roll and each of the both magnetic poles adjacent to that pole rise to the developing roll side so that the contact region of magnetic brush to the developing roll increases greatly. Consequently, when a bias for conveying toner onto the developing roll from the magnetic roll is applied, a predetermined saturated toner layer is instantaneously formed so that the image consistency of the top end of an image does not decrease, though it decreases often with conventional arts in case toner is resupplied to the developing roll after toner is recovered from the developing roll when an image is completely formed or developing begins, which results in providing an image forming apparatus having a hybrid developing apparatus giving stable image quality for a long time.
Further, according to the present invention, an intensity of magnetic force of the magnetic pole nearest to the developing roll on the magnetic roll is lower than an intensity of magnetic force of each of the both magnetic poles adjacent to that pole.
Thus, by setting an intensity of magnetic force of the magnetic pole nearest to the developing roll on the magnetic roll as lower than an intensity of magnetic force of each of the both magnetic poles adjacent to that pole, a comparatively wide developing nip of magnetic brushes can be formed along the circumference of the developing roll so that toner layer formation on the developing roll is effectively and rapidly made; therefore, the image consistency of the top end of an image does not decrease, though it decreases often with conventional arts in case toner is resupplied to the developing roll after toner is recovered from the developing roll when an image is completely formed or developing begins, which results in providing an image forming apparatus having a hybrid developing apparatus giving stable image quality for a long time.
Further, according to another aspect of the present invention, an image forming method comprising applying a developing bias for toner developing on a latent image bearing body to nearest approached regions (developing regions) between a developing roll and the latent image bearing body wherein an only toner thin layer is formed on the developing roll by utilizing a bias for conveying and magnetic brushes of the magnetic roll for conveying two components developer material which electrostatically charges developer material consisting of carrier and toner magnetically held thereon, is characterized in that a direct current bias is applied to the magnetic roll and a bias consisting of a direct current bias and an alternate current bias superposed thereon to the developing roll, and that sources of the direct current biases are independent from each other while a voltage of one of the direct current biases and a duty ratio of the alternate current bias are varied so as to refresh (recover and exchange) toner on the developing roll.
Thus, by applying direct current biases on the magnetic roll and the developing roll respectively with independent sources each other while varying a voltage of one of the direct current biases and a duty ratio of the alternate current bias, at least one thin layer on the developing roll is left while toner at the base part of the developing roll can be peeled off so that developing histories are erased while toner peeled off from the developing roll is recovered without agglomerating on the magnetic roll and is mixed with newly replenished toner resupplying smoothly to the developing roll. Therefore, generation of developing ghost and selective developing is prevented and surely charged toner is supplied to the developing roll without complicating the developing apparatus while unevenness of images is not generated even when successive printing, which results in providing an image forming apparatus having a hybrid developing apparatus giving stable image quality for a long time.
Further, according to the present invention, a duty ratio of the alternate current bias is from 50% to 70%.
Thus, by setting a duty ratio as from 50% to 70%, at least one thin layer on the developing roll is left when toner on the developing roll is recovered while toner at the base part of the developing roll can be peeled off so that a toner layer is formed which is capable of developing on the developing roll for the first round of the rotation of the developing roll without taking time for forming again a toner layer.
Further, according to the present invention, a gap between the developing roll and the magnetic roll is within the range of from 0.2 mm to 0.5 mm and a rotational direction of the developing roll is the same as the magnetic roll.
Thus, since a gap between the developing roll and the magnetic roll is within the range of from 0.2 mm to 0.5 mm, toner is transferred rapidly; since a rotational direction of the developing roll is the same as the magnetic roll, for example, when the developing roll rotates counterclockwise, the magnetic roll rotates also counterclockwise, toner is recovered favorably from the developing roll.
Further, according to the present invention, a period when developing is ceased for refreshing (recovering and exchanging) toner is a period between printing paper sheets (during a succession of paper sheets is being transported) and/or a period after printing (after a succession of paper sheets has been transported).
Thus, by refreshing toner during a period between printing paper sheets (during a succession of paper sheets is being transported) and/or a period after printing (after a succession of paper sheets has been transported), always adequate toner is supplied on the developing roll without affecting printing at all even if high consistent images are printed successively so that mottle in image consistency can be prevented while a sufficient printing rate is maintained. Hence, generation of ghost and poor performance for following solid images are prevented whereby providing an image forming method giving excellent images for a long time.
Further, according to the present invention, an image forming method comprising applying a developing bias for toner developing on a latent image bearing body to nearest approached regions (developing regions) between a developing roll and the latent image bearing body wherein an only toner thin layer is formed on the developing roll by utilizing a bias for conveying and magnetic brushes of the magnetic roll for conveying two components developer material which electrostatically charges developer material consisting of carrier and toner magnetically held thereon, is characterized in that a direct current bias is applied to the magnetic roll and a bias consisting of a direct current bias and an alternate current bias superposed thereon to the developing roll, and that sources of the direct current biases are independent from each other while, by varying one of the direct current biases, a part of toner on the developing roll is recovered during a period between printing paper sheets or printing data when the printing data is continuous and all the toner is recovered when the printing is completed.
Thus, since toner is recovered by varying one of the direct current biases in the state that a direct current bias is applied to the magnetic roll and a bias consisting of a direct current bias and an alternate current bias superposed thereon to the developing roll with independent sources of the direct current biases each other, a complicated structure such as an auxiliary electrode or buried electrodes in the developing roll is not necessary so that a bias according to the state of developing can be applied to the developing roll and the magnetic roll. Hence, an image forming method preventing fog and giving excellent images for a long time can be provided.
Further, according to the present invention, an image forming method comprising applying a developing bias for toner developing on a latent image bearing body to nearest approached regions (developing regions) between a developing roll and the latent image bearing body wherein an only toner thin layer is formed on the developing roll by utilizing a bias for conveying and magnetic brushes of the magnetic roll for conveying two components developer material which electrostatically charges developer material consisting of carrier and toner magnetically held thereon, is characterized in that a direct current bias is applied to the magnetic roll and a bias consisting of a direct current bias and an alternate current bias superposed thereon to the developing roll, that sources of the direct current biases are independent from each other, and that, by varying one or both of the direct current biases while a voltage value of the bias on the magnetic roll is maintained higher than a voltage value of the bias on the developing roll, toner on the developing roll is recovered so that a part of toner remains on the developing roll after developing.
Thus, since sources of the direct current biases are independent from each other, and by varying one or both of the direct current biases while a voltage value of the bias on the magnetic roll is maintained higher than a voltage value of the bias on the developing roll, toner on the developing roll is recovered so that a part of toner remains on the developing roll after developing, a complicated structure such as an auxiliary electrode or buried electrodes in the developing roll is not necessary and a toner thin layer is rapidly formed on the developing roll at the beginning of developing. Hence, an image forming method preventing decreasing of image consistency or fog and giving excellent images for a long time can be provided.
Further, according to the present invention of image forming method, a duty ratio of the alternate current bias is set as within the range from 10 to 50%.
Thus, by setting a duty ratio of the alternate current bias as within the range of 10 to 50%, a peak of an alternative current bias becomes sharp so that toner can be effectively recovered and recovery effect from the developing roll to the conveying body of developer material is enhanced so that toner staining of the developing roll is eliminated, and at the same time, toner is recovered effectively from the latent image bearing body to the developing roll so that fog is not likely to appear, whereby an image forming apparatus having a hybrid developing apparatus giving stable image quality can be provided.
Further, according to the present invention, an image forming method further comprises making up the surface of the developing roll with an approximately uniform electro conductive sleeve having a conductivity of 106 xcexa9xc2x7cm3 or less, and making an applied bias consisting of a direct current bias and an alternate current bias superposed thereon act between the developing roll and the electrostatic latent image bearing body and between the developing roll and the magnetic roll.
Thus, by making up the surface of the developing roll with an approximately uniform electro conductive sleeve having a conductivity of 106 xcexa9xc2x7cm3 or less, a bias consisting of a direct current bias and an alternate current bias superposed thereon applied on the developing roll acts appropriately between the rotating developing roll and the latent image bearing body and the magnetic roll. Hence, an image forming method preventing decreasing of image consistency even after a long use and giving stable image quality for a long time can be provided.
Further, according to the present invention, an image forming method further comprises making a ratio of a volume average particle size to a number average particle size of the toner which the developer material contains 1.25 or less and controlling an average particle size of the carrier as 50 xcexcm or less.
Thus, by making a ratio of a volume average particle size to a number average particle size of the toner which the developer material contains 1.25 or less and controlling an average particle size of the carrier 50 xcexcm or less, lowering of developing property owing to depositing finer particles of toner during successive printing is prevented avoiding increase of stress to carrier so as to stabilize an amount of toner supply to the developing roll and to allow sufficient charging to the toner in the developer material, which leads to stable developing.
Further, according to the present invention, an image forming method further comprises controlling an amount of charge of the toner which the developer material contains within the range of from 5 to 20 xcexcC/g.
Thus, by controlling an amount of charge of the toner which the developer material contains within the range of from 5 to 20 xcexcC/g, selective developing is prevented and a toner layer thickness is controlled to an optimum value so as to avoid toner scattering and no to lower developing property.
Further, according to the present invention, an image forming method comprises controlling a toner thickness to 50 xcexcm or less and a toner amount within range of from 0.5 to 1.7 mg/cm2.
Thus, by controlling a toner thickness to 50 xcexcm or less and a toner amount within range of from 0.5 to 1.7 mg/cm2 an image forming method without toner scattering and developing ghost can be provided.